Handle grep output

- Handle standard filepath:code and filepath:line_number:code output
  as produced by `git grep`, `rg -H`, `grep -H`, etc (with -n for line
  numbers).

- Retain the match highlighting as produced by the grep tool, and
  expose it in delta's color output styled with grep-match-style.
  (Note that --color=always is needed to retain the color if piping
  into delta, but not for `git grep` when delta is configured as git's
  pager)

- Special handling of -p, and -W options of `git grep`: these display
  the function context in which the matches occur.

- `navigate` keybindings jump between match function contexts under
  `git grep -p` and between matching lines under `git grep -W`.

Thanks @zachriggle for the proposal.
Fixes #769

src/cli.rs

@@ -441,6 +441,21 @@ pub struct Opt {
     )]
     pub blame_timestamp_format: String,
 
+    #[structopt(long = "grep-match-style")]
+    /// Style (foreground, background, attributes) for grep matches. See STYLES
+    /// section. Defaults to plus-style.
+    pub grep_match_style: Option<String>,
+
+    #[structopt(long = "grep-match-file-style")]
+    /// Style (foreground, background, attributes) for grep match file paths. See STYLES
+    /// section. Defaults to hunk-header-file-path-style.
+    pub grep_match_file_style: Option<String>,
+
+    #[structopt(long = "grep-match-line-number-style")]
+    /// Style (foreground, background, attributes) for grep match line numbers. See STYLES
+    /// section. Defaults to hunk-header-line-number-style.
+    pub grep_match_line_number_style: Option<String>,
+
     /// Default language used for syntax highlighting when this cannot be
     /// inferred from a filename. It will typically make sense to set this in
     /// per-repository git config (.git/config)

src/config.rs

@@ -78,6 +78,9 @@ pub struct Config {
     pub git_config: Option<GitConfig>,
     pub git_minus_style: Style,
     pub git_plus_style: Style,
+    pub grep_match_file_style: Style,
+    pub grep_match_line_number_style: Style,
+    pub grep_match_style: Style,
     pub hunk_header_file_style: Style,
     pub hunk_header_line_number_style: Style,
     pub hunk_header_style_include_file_path: bool,
@@ -216,6 +219,22 @@ impl From<cli::Opt> for Config {
             _ => *style::GIT_DEFAULT_PLUS_STYLE,
         };
 
+        let grep_match_style = if let Some(s) = opt.grep_match_style {
+            Style::from_str(&s, None, None, opt.computed.true_color, false)
+        } else {
+            plus_emph_style
+        };
+        let grep_match_file_style = if let Some(s) = opt.grep_match_file_style {
+            Style::from_str(&s, None, None, opt.computed.true_color, false)
+        } else {
+            hunk_header_file_style
+        };
+        let grep_match_line_number_style = if let Some(s) = opt.grep_match_line_number_style {
+            Style::from_str(&s, None, None, opt.computed.true_color, false)
+        } else {
+            hunk_header_line_number_style
+        };
+
         let blame_palette = make_blame_palette(opt.blame_palette, opt.computed.is_light_mode);
 
         let file_added_label = opt.file_added_label;
@@ -282,6 +301,9 @@ impl From<cli::Opt> for Config {
             file_style,
             git_config: opt.git_config,
             git_config_entries: opt.git_config_entries,
+            grep_match_file_style,
+            grep_match_line_number_style,
+            grep_match_style,
             hunk_header_file_style,
             hunk_header_line_number_style,
             hunk_header_style,

src/delta.rs

@@ -23,6 +23,7 @@ pub enum State {
     SubmoduleLog, // In a submodule section, with gitconfig diff.submodule = log
     SubmoduleShort(String), // In a submodule section, with gitconfig diff.submodule = short
     Blame(String, Option<String>), // In a line of `git blame` output (commit, repeat_blame_line).
+    Grep(String, Option<String>), // In a line of `git grep` output (file, repeat_grep_line).
     Unknown,
     // The following elements are created when a line is wrapped to display it:
     HunkZeroWrapped,  // Wrapped unchanged line
@@ -121,6 +122,7 @@ impl<'a> StateMachine<'a> {
                 || self.handle_submodule_short_line()?
                 || self.handle_hunk_line()?
                 || self.handle_blame_line()?
+                || self.handle_grep_line()?
                 || self.should_skip_line()
                 || self.emit_line_unchanged()?;
         }

src/handlers/file_meta.rs

@@ -210,7 +210,7 @@ fn get_file_extension_from_file_meta_line_file_path(path: &str) -> Option<&str>
 }
 
 /// Attempt to parse input as a file path and return extension as a &str.
-fn get_extension(s: &str) -> Option<&str> {
+pub fn get_extension(s: &str) -> Option<&str> {
     let path = Path::new(s);
     path.extension()
         .and_then(|e| e.to_str())